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108
the milk. Some of these bacteria may be harmless; indeed, we
know that some are advantageous and beneficial, assisting in the
ripening of cream and flavouring of butter. But others may be of
putrefactive species, bringing about decomposition in the milk to
which they gain access. And a third group may be actually
disease-producing species, conveying disease to the persons drinking
the milk. Hence it is clear that, by a bacteriological examination,
we may obtain facts of great value as to the condition of a milk,
and in any case form some opinion as to the degree of pollution by
dust and dirt which milk receives.
Now much of the market milk contains very large numbers of
bacteria. Dr. Park, of New York, has made extensive researches
into the quality of milk sold in that city, and he finds that the
number of bacteria present varies according to season and according
to the standard of milk shop. In a poor district the average
number of bacteria present out of 13 samples was 1,977,692 per
cubic centimetre (—15 drops) and in a well-to-do district the average
of 10 samples was 327,500 bacteria per c.c.* Similar investigations
have been made in Berlin, Paris, Edinburgh, Boston, Melbourne and
other cities. As a result it has come to be recognized that even
good milk may contain from 50,000 to 500,000 micro-organisms per
c.c., if such are not injurious species. Several of the American
Milk Commissions have laid down that 30,000 bacteria per c.c. is a
suitable standard by which to judge the quality of milk provided
none of these are disease producing. Such figures apply to freshly
drawn milk and not to milk which has travelled a long distance to
market. Certainly such a standard is, at present, too high for
London milks.
For the purposes of this report I have made a careful bacteriological
examination of nine London milks. Purchases were made
in five shops situated in Finsbury, and four shops located
in the neighbouring districts of the City of London, the City
of Westminster, and the Boroughs of Holborn and Islington.
Three out of the nine samples were purchased in small poor shops,
the other six in good class shops. The sample was bought
in the ordinary way, and it was not known that it was being
° Journal of Hygiene, 1901 (July) p. 395.